Priority is claimed under 35 U.S.C. § 119 to Japanese patent application nos. 2002-247403 filed Aug. 27, 2002, 2003-030816 filed Feb. 7, 2003 and 2003-194274 filed Jul. 9, 2003, all of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method for forming an optical waveguide in a transparent material such as a pure silica glass by focused femtosecond laser pulses. Also, the present invention relates to an optical waveguide which is formed according to the above method.
2. Description of Related Art
According to a method for forming an optical waveguide by focused femtosecond laser pulses, there is a phenomenon in which a refractive index increases near a focal point when high intensity femtosecond laser pulses are focused in various transparent materials such as a pure silica glass. By utilizing such a phenomenon, it is possible to form a refractive index change and a linear shape of refractive index increase region such as an optical waveguide by focused femtosecond laser pulses in a transparent material and scanning femtosecond laser pulses or the transparent material (See Japanese Unexamined Patent Application, First Publication No Hei 9-311237).
When forming such an optical waveguide according to this forming method, the induced refractive index distribution to be changed is determined according to various process conditions such as “average output power”, “pulse width”, “repetition rate”, “center wavelength” in femtosecond laser pulses which are focused, and other process conditions such as a numerical aperture of a microscope objective for focusing femtosecond laser pulses and a scanning speed of a precision stage.
Japanese Unexamined Patent Application, First Publication No 2001-350049, discloses that it is possible to change the refractive index distribution to be changed in an optical waveguide gradually by changing an average output power of femtosecond laser pulses which are focused so as to be synchronized with a movement of a precision stage on which a pure silica glass is mounted. As a result, it is possible to change a mode field diameter of an optical waveguide along with the average output power of femtosecond laser pulses which are changed. It is also possible to reduce a connection loss between an optical waveguide and an optical fiber.
Here, the intensity of the femtosecond laser pulses are not necessarily distributed circular at the focal point when femtosecond laser pulses are focused because of an influence of aberrations by a microscope objective. It is understood that a shape of refractive index increase region, which is induced, changes because of an intensity distribution of femtosecond laser pulses at the focal point. That is, it is understood that the refractive index distribution is not adequate when the intensity distribution of femtosecond laser pulses at the focal point does not become circular; thus the mode field of a guided light in an optical waveguide does not become circular.
Also, it is necessary to connect the optical waveguide. Usually, a refractive index distribution in an optical components such as an optical fiber is circular. Therefore, there is a problem in that a connection loss increases and a polarization mode dispersion and a polarization dependent loss increase unless a mode field in an optical waveguide is circular.
Furthermore, it is necessary that a mode field diameter of an optical waveguide and a mode field diameter of optical components, such as an optical fiber, are the same as each other or close to each other when an optical waveguide is connected to optical components such as an optical fiber. There is a problem in that a connection loss increases if a difference of the mode field diameter is large between them. It is possible to control the mode field diameter in an optical waveguide by changing an average output power of femtosecond laser pulses when an optical waveguide is formed by focused femtosecond laser pulses. It is reported, in a document “The Review of Laser Engineering, Miura et al., February Issue, pages 150 to 154”, that a mode field diameter increases when an average output power of femtosecond laser pulses increases.
To obtain an optical waveguide having a 7 μm of mode field diameter, which is a mode field diameter commonly used for a single mode fiber, by focusing femtosecond laser pulses, it is necessary to increase an average output power of femtosecond laser pulses.
However, when an optical waveguide is formed by increasing an average output power of femtosecond laser pulses, the mode field does not become circular. That is, the mode field becomes elliptical which expands in a direction in which femtosecond laser pulses are focused. Thus, there is above problem.
An object of the present invention is to control a mode field diameter of an optical waveguide when an optical waveguide is formed in a transparent material by focused femtosecond laser pulses. Also, other object of the present invention is to control an aspect ratio of a mode field diameter. Yet, other object of the present invention is to control both a mode field diameter and the aspect ratio of the mode field diameter. Also, an object of the present invention is to form a mode field in a range of 10 to 14 μm and an aspect ratio is in a range of 0.9 to 1.1 by controlling such process conditions.